An unsaturated polyester alkyd has hitherto been produced by condensing a mixture of an unsaturated polybasic acid such as maleic anhydride or fumaric acid and a saturated polybasic acid such as phthalic anhydride, with a polyhydric alcohol such as propylene glycol or ethylene glycol. Such an unsaturated polyester alkyd is cross-linked with an unsaturated monomer such as styrene to form a cured product which is useful as, for example, a construction material, casting and paint.
However, a commonly used unsaturated polyester resin, for example, one which is the most commonly used and which comprises 100 parts by weight of a polyester alkyd having a molecular weight of 1,200 to 2,200 which is prepared from maleic anhydride, phthalic anhydride and propylene glycol, and 60 to 150 parts by weight of a styrene monomer has a strong smell of the styrene monomer that causes a headache in molding operators. The evaporation of the vinyl monomer such as styrene can be reduced by blending a small amount of the vinyl monomer with the unsaturated alkyd. But, a composition comprising 100 parts by weight of the above-illustrated unsaturated alkyd and 60 parts by weight of a styrene monomer generally has a viscosity of about 500 cps at 25.degree. C., and to use even less styrene monomer results in a composition of higher viscosity which is difficult to be blended with glass fibers or inorganic fillers.
In the course of efforts to eliminate these defects of the conventional unsaturated polyester resin, it was noted that if the viscosity of the unsaturated polyester alkyd is low, then the amount of the vinyl monomer to be used can be reduced, and that hence, not only the smell of the vinyl monomer can be inhibited, but also the following problems due to the high viscosity of the unsaturated polyester alkyd can be solved. Based on this recognition, it was attempted to produce various unsaturated polyester alkyds to thereby find the desired low-viscosity alkyd.
(1) If the unsaturated polyester alkyd produced has high viscosity, the agitation becomes insufficient to cause decrease of the reaction rate and formation of a gel due to local heating; and
(2) The unsaturated polyester alkyd prepared is dissolved in an unsaturated monomer such as styrene at a temperature which is reduced to prevent gelation, but as the temperature becomes low, the viscosity is increased to reduce the solubility of the alkyd, and in consequence, lumps of the alkyd are formed which impairs the appearance of the cured product or decreases its impact strength. If the formation of alkyd lumps is avoided by increasing the amount of unsaturated monomer, only a brittle cured product results. The high viscosity of the alkyd is particularly disadvantageous in a winter season.
As a result of studies to solve these problems, it has been found that an unsaturated polyester alkyd of very low viscosity can be produced if 1,2-butanediol that is currently burnt away or discarded as a waste is used as the polyhydric alcohol component and phthalic anhydride as the polybasic acid.